System V ABI x64
                  ================


This document describes concisely the subset of the x64
as it is implemented in QBE.  The subset can handle
correctly arbitrary standard C-like structs containing
float and integer types.  Structs that have unaligned
members are also supported through "dark" types, see
the IR description document for more information about
them.


- ABI Subset Implemented
-----------------------------

Data classes of interest as defined by the ABI:
 - INTEGER
 - SSE
 - MEMORY


Classification:

1. The size of each argument gets rounded up to eightbytes.
   (It keeps the stack always 8 bytes aligned.)
2. _Bool, char, short, int, long, long long and pointers
   are in the INTEGER class.  In the context of QBE, it
   means that 'l' and 'w' are in the INTEGER class.
3. float and double are in the SSE class.  In the context
   of QBE, it means that 's' and 'd' are in the SSE class.
4. If the size of an object is larger than two eightbytes
   or if contains unaligned fields, it has class MEMORY.
   In the context of QBE, those are big aggregate types
   and "dark" types.
5. Otherwise, recursively classify fields and determine
   the class of the two eightbytes using the classes of
   their components.  If any is INTEGER the result is
   INTEGER, otherwise the result is SSE.

Passing:

 - Classify arguments in order.
 - INTEGER arguments use in order %rdi %rsi %rdx %rcx
   %r8 %r9.
 - SSE arguments use in order %xmm0 - %xmm7.
 - MEMORY gets passed on the stack.
 - When we run out of registers for an aggregate, revert
   the assignment for the first eightbytes and pass it
   on the stack.
 - When all registers are taken, write arguments on the
   stack from right to left.
 - When calling a variadic function, %al stores the number
   of vector registers used to pass arguments (it must be
   an upper bound and does not have to be exact).
 - Registers %rbx, %r12 - %r15 are callee-save.

Returning:

 - Classify the return type.
 - Use %rax and %rdx in order for INTEGER return values.
 - Use %xmm0 and %xmm1 in order for SSE return values.
 - I the return value's class is MEMORY, the first
   argument of the function %rdi was a pointer to an
   area big enough to fit the return value.  The function
   writes the return value there and returns the address
   (that was in %rdi) in %rax.


- Various Remarks
-----------------------------